This invention relates to radio communications equipment and more specifically to radio frequency (RF) mixers used in RF equipment.
Radio communications equipment use mixers to convert a signal from a low frequency to a high frequency or a high frequency to a low frequency by mixing the signal with a local oscillator signal. The local oscillator frequency can be above or below the frequency of the desire signal to produce a sum and a difference frequency one of which is the frequency of interest.
Mixer performance is critical to the overall performance of the RF equipment. Mixer performance is especially critical in receiver applications where the dynamic range of the receiver is limited by the mixer. Mixers must have good linearity or intermodulation performance characteristics, wide dynamic range, low power consumption, low noise figure, and offer conversion gain in some applications.
Direct conversion or homodyne receivers convert the RF signal directly to baseband or a zero intermediate frequency (IF). Mixers for this application require low second or even-order distortion products, good balance, and low 1/f noise. Diode ring mixers are not suitable for direct conversion receiver applications since they have poor balance, require high injection levels, and have a conversion loss. Active mixers such as the Gilbert cell fabricated in gallium arsenide (GaAs) heterojunction bipolar transistor (HBT) technology offer some performance advantages over passive diode mixers. However these active mixer designs are expensive and suffer from 1/f noise problems and other low frequency discrete noise spurs due to GaAs processing issues. These mixers also have high power consumption and larger than desired noise figures.
What is needed for direct conversion and other receiver applications is a mixer that has no anomalous low frequency spurs, has low distortion, has significantly reduced power consumption, lower noise figure, good balance, and low cost.